Neural consequences of chronic sleep disruption
Authors: Zachary Zamore, Sigrid C. Veasey
Journal: Trends in Neurosciences (2022)

Recent evidence in humans reveals that chronic sleep disruption can lead to protracted recovery of neurobehavioral performance, particularly sustained vigilance and episodic memory.Studies in animal models of chronic sleep disruption demonstrate protracted and even incomplete recovery, including neuron loss in brain areas critical for vigilance and episodic memory, specifically, the locus coeruleus and hippocampus.The severity of neural injury incurred by chronic sleep disruption varies with duration and type of sleep disruption, age at which sleep loss exposure occurs, neuronal populations being assessed, and genetic predisposition to neurodegenerative processes.Early oxidative stress and sustained inflammation contribute to a metabolic resetting, behavioral impairment, and pathologic findings associated with chronic sleep disruption. Recent studies in both humans and animal models call into question the completeness of recovery after chronic sleep disruption. Studies in humans have identified cognitive domains particularly vulnerable to delayed or incomplete recovery after chronic sleep disruption, including sustained vigilance and episodic memory. These findings, in turn, provide a focus for animal model studies to critically test the lasting impact of sleep loss on the brain. Here, we summarize the human response to sleep disruption and then discuss recent findings in animal models examining recovery responses in circuits pertinent to vigilance and memory. We then propose pathways of injury common to various forms of sleep disruption and consider the implications of this injury in aging and in neurodegenerative disorders. Recent studies in both humans and animal models call into question the completeness of recovery after chronic sleep disruption. Studies in humans have identified cognitive domains particularly vulnerable to delayed or incomplete recovery after chronic sleep disruption, including sustained vigilance and episodic memory. These findings, in turn, provide a focus for animal model studies to critically test the lasting impact of sleep loss on the brain. Here, we summarize the human response to sleep disruption and then discuss recent findings in animal models examining recovery responses in circuits pertinent to vigilance and memory. We then propose pathways of injury common to various forms of sleep disruption and consider the implications of this injury in aging and in neurodegenerative disorders. Chronic sleep disruption in humans: evolving thoughts on recoveryChronic sleep curtailment is common in modern society and is related in part to increased work demands, lifestyle choices, the development and use of medications and substances that suppress or disrupt sleep, in addition to the increased use of artificial light-emitting devices that delay sleep. A generally held presumption has been that while chronic sleep disruption results in neurobehavioral impairments, performance deficits are reversed with limited-period recovery sleep (e.g., over the weekend). A collection of studies examining the human response to sleep loss, however, challenges this belief and suggests that impairments persist, and that additionally, individuals may be poor judges of their incurred sleep loss impairments over time. From 1 to 2 weeks of sleep restriction to <7 h sleep/night in adults has been shown repeatedly to result in cumulative increases in sleep propensity, along with decrements in mood and vigilance [1.Dinges D.F. et al.Cumulative sleepiness, mood disturbance, and psychomotor vigilance performance decrements during a week of sleep restricted to 4-5 hours per night.Sleep. 1997; 20: 267-277PubMed Google Scholar, 2.Van Dongen H.P. et al.The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation.Sleep. 2003; 26: 117-126Crossref PubMed Scopus (2126) Google Scholar, 3.Belenky G. et al.Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: a sleep dose-response study.J. Sleep Res. 2003; 12: 1-12Crossref PubMed Scopus (979) Google Scholar]. Objective and subjective discrepancies, however, were evident, as study participants were unaware of the progressive deterioration in performance across sleep restriction [1.Dinges D.F. et al.Cumulative sleepiness, mood disturbance, and psychomotor vigilance performance decrements during a week of sleep restricted to 4-5 hours per night.Sleep. 1997; 20: 267-277PubMed Google Scholar,2.Van Dongen H.P. et al.The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation.Sleep. 2003; 26: 117-126Crossref PubMed Scopus (2126) Google Scholar]. While subjective impairments (sleepiness and mood) typically normalized with 1–2 nights of recovery sleep, objective measures of vigilance showed persistent deficits, relative to baseline performance, after 2–3 nights of recovery sleep [3.Belenky G. et al.Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: a sleep dose-response study.J. Sleep Res. 2003; 12: 1-12Crossref PubMed Scopus (979) Google Scholar, 4.Banks S. et al.Neurobehavioral dynamics following chronic sleep restriction: dose-response effects of one night for recovery.Sleep. 2010; 33: 1013-1026Crossref PubMed Google Scholar, 5.Pejovic S. et al.Effects of recovery sleep after one work week of mild sleep restriction on interleukin-6 and cortisol secretion and daytime sleepiness and performance.Am. J. Physiol. Endocrinol. Metab. 2013; 305: E890-E896Crossref PubMed Scopus (105) Google Scholar]. A similarly delayed recovery from sleep loss was evident in adolescents with incomplete recovery in sustained vigilance after 2 nights of recovery sleep [6.Lo J.C. et al.Neurobehavioral impact of successive cycles of sleep restriction with and without naps in adolescents.Sleep. 2017; 40zsw042Crossref Scopus (51) Google Scholar]. In young adults, sustained vigilance was impaired across 5 days of 4 h/night sleep restriction, and the increased frequency of lapses in performance was not reversed despite 3 recovery nights [7.Axelsson J. et al.Sleepiness and performance in response to repeated sleep restriction and subsequent recovery during semi-laboratory conditions.Chronobiol. Int. 2008; 25: 297-308Crossref PubMed Scopus (93) Google Scholar]. More recently, a field study was conducted in which sleep time was reduced by one third for 10 consecutive days in young adults [8.Ochab J.K. et al.Observing changes in human functioning during induced sleep deficiency and recovery periods.PLoS One. 2021; 16e0255771Crossref PubMed Scopus (0) Google Scholar]. Neither accuracy in a cognitive interference assay (Stroop) nor eyes-open alpha power spectra had normalized after a 7-day recovery period [8.Ochab J.K. et al.Observing changes in human functioning during induced sleep deficiency and recovery periods.PLoS One. 2021; 16e0255771Crossref PubMed Scopus (0) Google Scholar]. When adults were deprived of all sleep for 40 consecutive hours, subjective sleepiness resolved after 1 night of recovery sleep, while performance of tasks requiring higher cognitive function (e.g., reading comprehension, serial addition, and go-no-go tasks) did not normalize after 2 recovery nights [9.Ikegami K. et al.Recovery of cognitive performance and fatigue after one night of sleep deprivation.J. Occup. Health. 2009; 51: 412-422Crossref PubMed Scopus (28) Google Scholar]. As shown for chronic sleep restriction, adults across 3 days of recovery after total sleep deprivation overestimate their own vigilance performance [10.Boardman J.M. et al.The ability to self-monitor cognitive performance during 60 h total sleep deprivation and following 2 nights recovery sleep.J. Sleep Res. 2018; 27e12633Crossref PubMed Scopus (12) Google Scholar]. In addition to impaired vigilance, one night of total sleep deprivation impairs both episodic memory and hippocampal connectivity to the prefrontal cortex and default mode network [11.Chai Y. et al.Two nights of recovery sleep restores hippocampal connectivity but not episodic memory after total sleep deprivation.Sci. Rep. 2020; 10: 8774Crossref PubMed Scopus (14) Google Scholar]. While 2 nights of recovery sleep restores hippocampal connectivity, episodic memory impairments persists. Collectively, these lines of work strongly support the notion that humans are vulnerable to protracted recovery responses after chronic sleep disruption and manifest imperceptions of the incurred impairments. The latter finding may have contributed to the false sense of security upon weekend recovery sleep.Neurobehavioral deficits in response to sleep disruption in both humans and animal models have been largely attributed to homeostatic wake-induced increases in brain extracellular adenosine; levels of which readily reverse upon recovery sleep after short-term sleep loss [12.Porkka-Heiskanen T. et al.Adenosine: a mediator of the sleep-inducing effects of prolonged wakefulness.Science. 1997; 276: 1265-1268Crossref PubMed Scopus (895) Google Scholar, 13.Kalinchuk A.V. et al.The time course of adenosine, nitric oxide (NO) and inducible NO synthase changes in the brain with sleep loss and their role in the non-rapid eye movement sleep homeostatic cascade.J. 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In a study on individuals undergoing 40 h of total sleep deprivation, extracellular adenosine did not increase in any of the brain regions in which measurements were made, including the hippocampus [17.Zeitzer J.M. et al.Extracellular adenosine in the human brain during sleep and sleep deprivation: an in vivo microdialysis study.Sleep. 2006; 29: 455-461Crossref PubMed Scopus (33) Google Scholar]. In this study, however, participants had histories of epilepsy, and values were not compared to samples in individuals allowed to sleep to determine expected drift in adenosine measures over time. In rats exposed to 3 days of sleep restriction, adenosine levels measured in hippocampal slices were reduced, rather than elevated, and after a 2-week recovery opportunity, levels in slice remained below baseline levels 2 weeks into recovery [18.Clasadonte J. et al.Chronic sleep restriction disrupts sleep homeostasis and behavioral sensitivity to alcohol by reducing the extracellular accumulation of adenosine.J. Neurosci. 2014; 34: 1879-1891Crossref PubMed Scopus (51) Google Scholar]. By contrast, after 4 h of sleep deprivation, adenosine levels increased as expected in hippocampal slices [18.Clasadonte J. et al.Chronic sleep restriction disrupts sleep homeostasis and behavioral sensitivity to alcohol by reducing the extracellular accumulation of adenosine.J. Neurosci. 2014; 34: 1879-1891Crossref PubMed Scopus (51) Google Scholar]. Thus, adenosine may contribute to sleepiness and performance decrements caused by acute sleep loss. However, the presence of incomplete recoveries from chronic partial sleep loss in the absence of evidence of protracted elevations of adenosine in chronic sleep disruption, raises the possibility of sleep-loss-induced neural injury.Neural injury in response to sleep loss has been difficult to assess without predefined indices indicative of lasting neuronal loss, glial modification, and/or dysfunction. This challenge is illustrated by a series of experiments performed over three decades ago, which aimed to assess potential brain cell damage in rats following sleep deprivation using methodologies available at the time. Adult rats were exposed to an extreme form of sleep deprivation: total sleep deprivation for 2–3 weeks. The sleep deprivation resulted in profound systemic changes (severe weight loss, malnutrition, bacterial sepsis, hormonal dysregulation, and ultimately death), yet examination of the brains immediately after sleep loss for general histology, apoptosis, and necrosis, yielded no significant brain abnormalities [19.Cirelli C. et al.No evidence of brain cell degeneration after long-term sleep deprivation in rats.Brain Res. 1999; 840: 184-193Crossref PubMed Scopus (0) Google Scholar]. Does this mean that sleep loss does not injure the brain? We would argue that when injury is defined as neuronal loss or lasting behavioral impairment or circuit dysfunction, additional analyses are needed to exclude sleep loss neural injury.With improved definitions, assays and strategies to assess neural injury following sleep loss, a paradigm shift is emerging, with a transition from considering the effects of sleep loss as a readily reversible response, to greater appreciation that sleep loss can result in lasting neuron loss and dysfunction. In this Review, we begin by highlighting recent findings in animal models, emphasizing the more protracted neural effects of sleep disruption. We discuss how impairment and reversibility are influenced, not only by the chronicity of the sleep disruption but also by the age at the time of exposure, the interval after sleep loss used for assessment, neuronal subtypes or brain regions, and the propensity for protein aggregation. We then review what has been learned regarding mechanisms of neural injury following sleep loss and suggest future directions to ultimately identify therapies to lessen brain injury incurred during the commonly encountered scenarios of sleep disruption.Neural effects of chronic sleep disruption in animal modelsConsideration of sleep disruption paradigmsVarious modalities of sleep disruption have been developed to determine the effects of total sleep deprivation, sleep restriction, rapid eye movement (REM) sleep deprivation, and sleep fragmentation. There is some inherent overlap across modes of sleep disruption. For example, total sleep deprivation, by definition, includes REM sleep deprivation, and methods of REM sleep deprivation can impart partial disruption of non-REM (NREM) sleep [20.Arthaud S. et al.Paradoxical (REM) sleep deprivation in mice using the small-platforms-over-water method: polysomnographic analyses and melanin-concentrating hormone and hypocretin/orexin neuronal activation before, during and after deprivation.J. Sleep Res. 2015; 24: 309-319Crossref PubMed Scopus (28) Google Scholar,21.Silva R.H. et al.Role of hippocampal oxidative stress in memory deficits induced by sleep deprivation in mice.Neuropharmacology. 2004; 46: 895-903Crossref PubMed Scopus (250) Google Scholar]. A simple technique of platform-over-water may be used to prevent all sleep or REM sleep, depending on the size of the platforms relative to the size of animals studied. The platform size should be adjusted so that when animals initiate the sleep state to be disrupted, state-dependent reductions in postural muscle tone result in the animal falling from the platform into water and abruptly waking up. Larger platforms have been used to control for the platform environment; however, animals on these control platforms can show partial sleep restriction [22.Machado R.B. et al.Sleep deprivation induced by the modified multiple platform technique: quantification of sleep loss and recovery.Brain Res. 2004; 1004: 45-51Crossref PubMed Scopus (285) Google Scholar]. Platform approaches in some but not all studies result in elevated plasma corticosterone levels [20.Arthaud S. et al.Paradoxical (REM) sleep deprivation in mice using the small-platforms-over-water method: polysomnographic analyses and melanin-concentrating hormone and hypocretin/orexin neuronal activation before, during and after deprivation.J. Sleep Res. 2015; 24: 309-319Crossref PubMed Scopus (28) Google Scholar,23.Tobler I. et al.The effect of sleep deprivation and recovery sleep on plasma corticosterone in the rat.Neurosci. Lett. 1983; 35: 297-300Crossref PubMed Google Scholar,24.Suchecki D. et al.Increased ACTH and corticosterone secretion induced by different methods of paradoxical sleep deprivation.J. Sleep Res. 1998; 7: 276-281Crossref PubMed Google Scholar]. Additionally, each of these methods disrupts the continuum of NREM and REM sleep patterns across a 24-h cycle.Most studies to date, including those discussed here, have implemented physical means to disrupt sleep, although it is now possible to elicit sleep disruption with chemogenetic activation of wake circuits [25.Holth J.K. et al.The sleep-wake cycle regulates brain interstitial fluid tau in mice and CSF tau in humans.Science. 2019; 363: 880-884Crossref PubMed Scopus (252) Google Scholar] or, potentially, inactivation of sleep circuits. Commonly used paradigms for the various types of targeted sleep disruption are illustrated in Figure 1, along with potential advantages, disadvantages, and confounding factors. Additionally, there are differences in the level of arousal and the amount of learning experienced across methods of total sleep deprivation, where paradigms of exploratory wakefulness induce more robust immediate early gene response in wake-activated neurons, relative to use of gentle handling across the same duration [26.Deurveilher S. et al.Social and environmental contexts modulate sleep deprivation-induced c-Fos activation in rats.Behav. Brain Res. 2013; 256: 238-249Crossref PubMed Scopus (8) Google Scholar]. Exploratory wakefulness results in increased locomotor activity, and by providing a continuously enriched and changing environment, exploratory wakefulness may involve more learning than other approaches with constant (or expected) environments. Techniques that require continuous experimenter vigilance to disrupt sleep, like gentle handling, may allow more breakthrough sleep than automated systems (e.g., rotating platform) that intervene as soon as sleep electroencephalographic patterns are detected by a computer algorithm [27.Leenaars C.H. et al.A new automated method for rat sleep deprivation with minimal confounding effects on corticosterone and locomotor activity.J. Neurosci. Methods. 2011; 196: 107-117Crossref PubMed Scopus (0) Google Scholar,28.Deboer T. et al.Long term effects of sleep deprivation on the mammalian circadian pacemaker.Sleep. 2007; 30: 257-262Crossref PubMed Google Scholar]. Sleep fragmentation can be elicited with either a bar sweeping across the floor of the cage at set intervals or by way of a rotor table that briskly moves cages for a fraction of every minute or two. Methods to fragment sleep can also shorten 24-h sleep amounts early on in the exposure [29.Sinton C.M. et al.Validation of a novel method to interrupt sleep in the mouse.J. Neurosci. Methods. 2009; 184: 71-78Crossref PubMed Scopus (0) Google Scholar]. Sleep fragmentation by either approach is generally effective for long continuous periods (weeks) of sleep disruption and seems not to influence either corticosterone levels or body weight [30.Li Y. et al.Effects of chronic sleep fragmentation on wake-active neurons and the hypercapnic arousal response.Sleep. 2014; 37: 51-64Crossref PubMed Scopus (0) Google Scholar,31.Wallace E. et al.Differential effects of duration of sleep fragmentation on spatial learning and synaptic plasticity in pubertal mice.Brain Res. 2015; 1615: 116-128Crossref PubMed Scopus (15) Google Scholar]. The rotor table technique for sleep fragmentation allows group housing, ad libitum eating and drinking, and access to undisturbed nests. By contrast, in the sweeper bar technique nests cannot be maintained, and the animals are not group-housed. As each sleep disruption approach has its own strengths and limitations, we focus largely on sleep disruption findings for which use of multiple techniques have provided similar results.Locus coeruleus: an early hitDelayed recovery of sustained vigilance in the human studies supports the concept that chronic sleep disruption could affect neurons within the vigilance circuit, including locus coeruleus and anterior cingulate cortical neurons [32.Gompf H.S. et al.Locus ceruleus and anterior cingulate cortex sustain wakefulness in a novel environment.J. Neurosci. 2010; 30: 14543-14551Crossref PubMed Scopus (109) Google Scholar]. Locus coeruleus neurons (LCn) are wake-activated noradrenergic pontine neurons with firing rates highest during unexpected uncertainties [32.Gompf H.S. et al.Locus ceruleus and anterior cingulate cortex sustain wakefulness in a novel environment.J. Neurosci. 2010; 30: 14543-14551Crossref PubMed Scopus (109) Google Scholar,33.Bliss-Moreau E. et al.Anterior cingulate cortex ablation disrupts affective vigor and vigilance.J. Neurosci. 2021; 41: 8075-8087Crossref PubMed Scopus (4) Google Scholar]. While firing rates do not change across prolonged wakefulness, the duration of wakefulness influences metabolic responses in LCn. Upon brief wakefulness (3 h) LCn in adult mice upregulate mitochondrial deacetylase sirtuin type 3 (SirT3), which then initiates a mitochondrial antioxidant response to maintain redox homeostasis in LCn [34.Zhang J. et al.Extended wakefulness: compromised metabolics in and degeneration of locus ceruleus neurons.J. Neurosci. 2014; 34: 4418-4431Crossref PubMed Scopus (0) Google Scholar] (Figure 2) . However, when sleep loss is prolonged to 8 h of wakefulness/day for 3 consecutive days in the same enriched environment paradigm, SirT3 and its activating enzymes are not upregulated and LCn develop oxidative stress, increased mitochondrial protein acetylation, including acetylation and inactivation of electron transport chain proteins, and LCn counts are reduced [34.Zhang J. et al.Extended wakefulness: compromised metabolics in and degeneration of locus ceruleus neurons.J. Neurosci. 2014; 34: 4418-4431Crossref PubMed Scopus (0) Google Scholar]. Whether these changes observed immediately after the longer duration of sleep loss represent adaptation or neuronal dysfunction and/or injury, requires examination at a time point further from the termination of chronic sleep disruption. For example, the observed loss of LCn outlined above could represent an adaptive response in LCn to temporarily downregulate mitochondrial activity and tyrosine hydroxylase; the marker used for LCn cell counts in the forementioned study. Of note, one of the noradrenaline metabolites, 3,4-dihydroxyphenylglycoaldehyde, modifies tau, increasing its propensity to aggregate and propagate [35.Kang S.S. et al.Norepinephrine metabolite DOPEGAL activates AEP and pathological Tau aggregation in locus coeruleus.J. Clin. Invest. 2020; 130: 422-437Crossref PubMed Scopus (26) Google Scholar]. Thus, reducing tyrosine hydroxylase when faced with metabolic stress may be an adaptive response in chronic sleep restriction. According to this scenario, once metabolic homeostasis is reinstated, neuronal counts and sirtuin activity are expected to resume. However, when this enriched wakefulness paradigm was continued across 12 weeks, and mice were then allowed 1 year to normalize metabolics and counts, LCn counts remained reduced [36.Owen J.E. et al.Late-in-life neurodegeneration after chronic sleep loss in young adult mice.Sleep. 2021; 44zsab057Crossref Scopus (7) Google Scholar], indicating irreversible injury to LCn. Similarly, whole-cell patch clamp LCn recordings showed reduced frequency responses to step currents and increased after-hyperpolarization amplitudes 2 weeks after sleep fragmentation exposure, supporting delayed functional recovery [37.Li Y. et al.Effects of chronic sleep fragmentation on wake-active neurons and the hypercapneic arousal response.Sleep. 2014; 37: 51-64Crossref PubMed Scopus (0) Google Scholar]. In addition, sleep fragmentation in mice for 4 weeks results in a loss of LCn and a loss of SirT3 in remaining LCn, which is evident 4 weeks after sleep disruption, supporting the notion of lasting LCn injury and loss in response to chronic sleep fragmentation [38.Zhu Y. et al.Degeneration in arousal neurons in chronic sleep disruption modeling sleep apnea.Front. Neurol. 2015; 6: 109Crossref PubMed Scopus (28) Google Scholar]. Not all sleep disruption studies, however, have shown loss of LCn. One study in rats, for instance, used a rotating drum paradigm that alternated 3 h of enforced ambulation with 1 h rest, for 4 days/week during 4 weeks. In this paradigm, LCn loss was not observed relative to cell counts in rats in nonrotating drums [39.Deurveilher S. et al.No loss of orexin/hypocretin, melanin-concentrating hormone or locus coeruleus noradrenergic neurons in a rat model of chronic sleep restriction.Eur. J. Neurosci. 2021; 54: 6027-6043Crossref PubMed Scopus (0) Google Scholar]. LCn count variability was high in controls, but it is also conceivable that the brevity of each sleep disruption period (3 h) prevented LCn injury, and even engaged a SirT3 protective response across periods of brief wakefulness. In addition to loss of LCn, there is evidence of reduced LCn noradrenergic output in adult rats in response to chronic sleep loss. Rats exposed to 3 weeks of sleep fragmentation using a sweeping bar have reduced levels of extracellular noradrenaline in the hippocampus in response to a spatial learning task, relative to control animals [40.Lu H.J. Lv J. β-Adrenergic receptor activity in the hippocampal dentate gyrus participates in spatial learning and memory impairment in sleep-deprived rats.Exp. Neurobiol. 2021; 30: 144-154Crossref PubMed Google Scholar]. Because levels were measured immediately after sleep disruption, it is unclear whether this reduction in noradrenaline represents persistent dysfunction.Figure 2Duration-dependent effects of sleep loss on locus coeruleus neurons (LCn).Show full captionShort-term wakefulness [awake for three consecutive hours across the habitual sleep (lights-on) period] upregulates mitochondrial sirtuin type 3 (SirT3) activity, which then results in nuclear translocation of FoxO3a and transcriptional activation of antioxidants and PGC-1α to enhance mitochondrial biogenesis. By contrast, extended wakefulness (for 8 h a day in the lights-on period for three consecutive days) reduces SirT3 protein and its NAD+-synthesizing enzymes, thereby reducing SirT3 activity and increasing mitochondrial acetylation of (and thereby inactivating) antioxidant enzymes, electron transport chain proteins, and FoxO3a. Mechanisms by which LCn switch from an adaptive to maladaptive response are not known.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Why would LCn be vulnerable to increased mitochondrial stress in response to chronic sleep disruption? The metabolic status of LCn may dictate vulnerability to sleep loss. Mice deficient in SirT3 have lower LCn counts at baseline, yet counts do not decline further with prolonged sleep loss [34.Zhang J. et al.Extended wakefulness: compromised metabolics in and degeneration of locus ceruleus neurons.J. Neurosci. 2014; 34: 4418-4431Crossref PubMed Scopus (0) Google Scholar], suggesting that LCn with higher mitochondrial metabolic activity are more susceptible to loss upon extended waking. A second possibility is that exploratory wakefulness further raises LCn firing rates because of repeated exposures to unexpected uncertainties [41.Vankov A. et al.Response to novelty and its rapid habituation in locus coeruleus neurons of the freely exploring rat.Eur. J. Neurosci. 1995; 7: 1180-1187Crossref PubMed Scopus (201) Google Scholar]. LCn are most active in response to novelty or immediately upon arousal from sleep when autoreceptor activity is lowest, particularly in REM sleep [42.Aston-Jones G. Bloom F.E. Activity of norepinephrine-containing locus coeruleus neurons in behaving rats anticipates fluctuations in the sleep-waking cycle.J. Neurosci. 1981; 1: 876-886Crossref PubMed Google Scholar]. Opening of l-type calcium channels on LCn, upon arousal, or across extended periods of exploratory wakefulness is expected to increase calcium in mitochondria which would then activate calcium-dependent mitochondrial nitric oxide synthesis, impairing electron transport and increasing superoxide production in mitochondria [43.Sanchez-Padilla J. et al.Mitochondrial oxidant stress in locus coeruleus is regulated by activity and nitric oxide synthase.Nat. Neurosci. 2014; 17: 832-840Crossref PubMed Scopus (92) Google Scholar]. While identification of the mechanisms underlying LCn vulnerability requires further work (see Outstanding questions), we propose that the vulnerability of LCn is determined in part by the underlying metabolic activity of LCn, the mode and duration of sleep disruption implemented, and the ability of recovery sleep to resolve the metabolic perturbance.Hippocampal responses to chronic sleep disruptionIn humans, chronic sleep disruption impairs hippocampal-dependent episodic memory, and deficits persist after 2 nights of recovery sleep [11.Chai Y. et al.Two nights of recovery sleep restores hippocampal connectivity but not episodic memory after total sleep deprivation.Sci. Rep. 2020; 10: 8774Crossref PubMed Scopus (14) Google Scholar]. Th